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Abstract

Optical frequency combs serve as the clockwork of optical clocks, which are now the best time-keeping systems in existence. The use of precise optical time and frequency technology in various applications beyond the research lab remains a significant challenge, but one that integrated microresonator technology is poised to address. Here, we report a silicon-chip-based microresonator comb optical clock that converts an optical frequency reference to a microwave signal. A comb spectrum with a 25 THz span is generated with a 2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium frequency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clock’s output is the electronically countable 33 GHz microcomb line spacing, which features stability better than the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of tools needed for interfacing microcombs to state-of-the-art optical clocks.

Figures (4)

Microcomb optical clock with Rb atoms. (a) An intensity-modulated pump laser excites a chip-based microresonator (see micrograph at right) to create a 33 GHz spacing comb. The comb is broadened in highly nonlinear fiber (HNLF) following amplification to 1.4 W. Two lines of the comb 108 modes apart are stabilized to Rb transitions by control of the pump frequency and the intensity modulation feo. The clock output is obtained via photodetection of the unbroadened spectrum. Not shown are polarization controllers, which are needed before the intensity modulator, the microresonator, the HNLF, and all the elements of the Rb spectrometers. Other components are an optical bandpass filter (BPF), a bandreject filter (BRF), and two erbium-doped fiber amplifiers (EDFA). (b) Optical spectrum after a filter to suppress the pump (top) and following spectral broadening (bottom), (c) optical clock output over 12 h. Each point is the average of twenty 1 s measurements. For comparison, published Rb spectroscopic data on the D2–D1 difference divided by 108 has been subtracted. The solid [25] and hatched [26] gray regions represent previous data.

Injection locking to create an equidistant microcomb. (a) Model for microcomb generation, including a subcomb (green) and parametric seeding (red), (b) model RF spectrum after photodetection. All the comb lines contribute at frequency Δv=feo, and the presence of a subcomb is reflected in the sidebands spaced by v0. (c) Measured RF spectra with a 100 kHz resolution bandwidth, indicating a subcomb (top, green) with v0≈4MHz at feo−flock=−387kHz and an injection-locked comb (bottom, black) at feo=flock, (d) waterfall plot compiled from many traces like those in (c). The false color bar shows the scaling of RF power.